1. Field of the Invention
The present invention relates to an illumination light source that employs a coherent light source, and, more particularly to an illumination light source in which a speckle noise that is caused by coherence of a light source is reduced, and an image display apparatus that employs this illumination light source.
2. Description of the Related Art
In recent years, there are many cases in which a laser is used as a light source of optical image display apparatuses. Generally, light emitted from a laser has high directivity, and therefore, light use efficiency is expected to be improved. In addition, a monochromatic property thereof realizes a wide color reproduction area that is required in image display apparatuses. Therefore, lasers are considered to be useful as an illumination light source. On the other hand, because a laser beam has high coherence, if a laser is used as a light source of an image display apparatus, there is a problem that dot patterns called a speckle noise due to interference of light is caused. This is caused because minute irregularity in devices in an illumination optical system, light bulbs, devices in a projection optical system, or on a screen causes a phase of each of light beams that have passed through different points on a device surface to shift for an amount corresponding to the irregularity, and those light beams that are coherent with each other form an interference pattern on an image surface. Because surface precision of these devices is limited, when a light source having high coherence is used, influence of the speckle noise is always a problem. With the recent advance in a laser technology, development of a semiconductor laser that is compact and high-power, and that outputs high quality beams, and the like is active, and it is expected that a laser is increasingly used as a light source of image display apparatuses in future. Therefore, establishment of a speckle noise suppression method is demanded.
The problem of speckles in image display apparatuses in which a laser is used as a light source can be solved by reducing coherence of a laser beam while maintaining the property of the laser beam. As a method of reducing coherence of laser beams, conventionally, a method in which a plurality of laser beams that are incoherent with each other are synthesized to reduce the coherence of light as a whole has been widely used. This method is realized, for example, by a method in which a coherent light beam that is generated by a single laser (resonator) is divided into a plurality of light beams that are incoherent with each other and the light beams are synthesized, or by a method in which a plurality of coherent light beams incoherent with each other that are generated by different lasers (resonators) are synthesized.
As a technique based on the former method, a technique in which a plurality of juxtaposed element lenses (microlens array) are used (see, e.g., Japanese Patent Application Laid-open No. 2000-268603) and a technique in which a plurality of fiber bundles composed of optical fiber bundles having different lengths are used (see, e.g., Japanese Patent Application Laid-open No. H11-326653) have been proposed. In these techniques, light emitted from a laser is divided into a plurality of light beams using those devices to make a difference in an optical distance between the light beams large. Thus, the light beams become incoherent with each other, thereby reducing coherence of a light source as a whole.
According to the techniques, a plurality of light beams that are incoherent with each other can be generated by a single coherent light source, and the number of the light beams can be increased by changing a configuration of the devices. Therefore, coherence of the light source can be sufficiently reduced. However, because light loss is large in light division using a microlens array or fiber bundles, high light use efficiency is not achieved. In addition, to further reduce the coherence, a great number of fibers and light division devices are required. In terms of these problems, such techniques are not suitable for image display apparatuses that are required to have a high image quality in color or brightness, and for which miniaturization of an illumination device and cost reduction are demanded.
Furthermore, in both the former and the latter methods, to obtain low-coherence light, a plurality of light beams that are obtained by division or that are emitted from different lasers are required to be synthesized in such a manner that the optical axes thereof coincide with each other. In conventional light source devices, a plurality of coherent light beams are input to a subsequent optical system without completely matching the optical axes thereof. Therefore, there is a problem that spatial overlap is small due to difference in a propagation angle caused by each of the coherent light beams passing at different positions of the optical devices. Although these coherent light beams can be arranged to have completely the same light path by using a light synthesizing device, such a device that can synthesize light beams having the same wavelength and the same polarization without any light loss is not available. If a beam splitter is used to synthesize light beams, for example, only 50% of light use efficiency is expected per light. Although light synthesis without any light loss is possible if polarization of light and a polarization beam splitter are used, it is not practical to use a plurality of polarization beam splitters and polarization rotation devices.
As described above, the conventional speckle reducing methods still have problems to be solved in terms of light use efficiency in devices, cost, and miniaturization of apparatuses.